This invention relates generally to methods and systems for the electronic surveillance of articles, and especially to such systems and methods in which a tag circuit is attached to the articles and is electronically detected if the article is taken past an electronic surveillance station without prior removal or deactivation of the tag circuit.
Electronic article surveillance systems are known in which a so-called tag circuit is secured to the goods in a controlled area on the protected premises, and an exit detector is provided adjacent an exit from the controlled area, past which the article must normally be taken in exiting the premises. The exit detector electronically senses the presence of an active tag circuit on the article, and produces an alarm announcing the unauthorized removal of the article. To effect an authorized removal, the article with tag circuit is normally taken to a check station manned by authorizing personnel, such as a cashier for example, where the tag circuit is removed and/or deactivated in one way or another; subsequent removal of the article past the exit detection system then results in no alarm, as is desired.
If the tag circuit is removed at the check station, it is generally deactivated either permanently for discard, or temporarily for subsequent re-activation and re-use. If the tag circuit is disposable, i.e. can economically be discarded after one use, it can be left attached to the goods when they leave the check station, but in such case the tag circuit should be deactivated before it reaches the exit detector lest it cause a false alarm.
Various methods for deactivating tag circuits have been devised, depending in part upon the nature of the tag circuit. The present invention will be described with particular regard to a tag circuit which is resonant at least at one frequency, so that the presence of the tag circuit can be detected by transmitting signals at and/or near the tag-circuit resonant frequency, and by detecting changes in the signals produced at an adjacent receiver by the resultant resonance in the tag circuit.
It is known to deactivate such a tag circuit by transmitting to it signals at or near the resonant frequency of the tag circuit, at a power level sufficiently high to destroy the original resonant characteristic of the tag circuit. In some cases this may be accomplished by transmitting sufficient power to melt a fusible link in the tag circuit, thereby to render the circuit non-resonant or resonant at a substantially different frequency than the original resonant frequency, whereby its presence will not be detected by the exit detector and false alarms will be avoided. U.S. Pat. No. 4,567,473 of George J. Lichtblau, issued Jan. 28, 1986 describes a system in which the tag circuit is provided with specially thinned regions in a fusible insulator which normally separates, and insulates from each other, active portions of the tag circuit; when high-powered signals at the original resonant frequency of the tag circuit are transmitted to the tag circuit, an arc is formed through the insulator at the thinned region, and the metal of the tag circuit is thereby caused to extend through the resultant fused region to form a short-circuit which destroys, or greatly changes, the resonant characteristic of the tag circuit.
Unfortunately, due to production variations and to the effects of environmental conditions, the resonant frequencies of different tag circuits are typically not all the same. To accommodate this, it is known to sweep the frequency of the transmitter signals repetitively over a substantial range which includes all of the frequencies at which the various tag circuits are likely to be resonant. This phase of operation, during which the frequency is swept to detect presence of a tag circuit, is commonly designated as the search phase or mode. Similarly, it is common to sweep the frequency of the high-powered deactivating signals through the same search range so as to be sure to transmit deactivation signals at the resonant frequency of the tag circuit as required to deactivate the tag circuit.
While such a system is operable and has been used commercially, use of frequency-swept high-power deactivation signals has several drawbacks. First, it requires and wastes substantial amounts of transmitter power, since only the signals at or near the resonant frequency of the tag circuit serve any useful purpose. Secondly, steady transmission of the high-powered, swept-frequency deactivation signals produces unnecessary electrical radiations over a wide frequency band which tend to interfere with operation of other electrical apparatus and which may, in fact, be contrary to Government regulations or laws if transmitted with sufficient power to deactivate the tag circuit.
To minimize these undesirable effects, it is known to use low-power transmissions not only to determine that a tag circuit is present but also to determine what its resonant frequency is, and then to transmit the high-power deactivation signals substantially only at frequencies within the resonance frequency band of the tag circuit. This reduces the total energy needed for deactivation, and also reduces the extent of radio frequency interference.
One such system is described in the above-noted U.S. Pat. No. 4,567,473 of Lichtblau, wherein a digital source produces a staircase of voltage which is applied to a voltage-controlled oscillator (VCO) to produce a corresponding staircase sweep of the frequency of the transmitted tag-search signals. Upon the transmission of a frequency equal to or near the resonant frequency of the tag circuit, a sudden drop in transmitter antenna current occurs, which drop in current is sensed and used to identify the voltage step in the staircase which produced the resonant frequency; thereafter, the frequency sweep is discontinued, and the identified step value of voltage is applied steadily to the VCO to transmit the resonant frequency, continuously, at a level high enough and for a time long enough, to cause deactivation of the tag circuit. Since the deactivation signal is transmitted substantially only at the resonant frequency of the tag circuit, much less deactivation signal energy need be transmitted, producing savings in transmitter apparatus and power, as well as much less spectrum interference than when frequency-swept deactivation signals are used.
One known way to produce the high-powered deactivation transmission is to provide a power amplifier and a relatively high-power supply source for operating the power amplifier, and to switch the power amplifier into the transmitter signal path only when the deactivation level of signal is to be transmitted; at other times, the power amplifier is switched out of the circuit, so that only a low level of transmitted signal, suitable for the tag search operation, is transmitted at such times. Alternatively, the same amplifier may be used during both the high-power deactivation transmissions and the low-power search transmissions by appropriately changing its operating conditions to change its output power level.
While operative for its intended purposes, such a system normally requires a high-power supply source for the power amplifier, with attendant cost, bulk and heat-dissipation requirements, in order to generate deactivation transmissions of high enough intensity to destroy the resonant characteristics of any tag circuit which is close enough to the transmitter antenna to be detected during the search operation.
Accordingly, an object of this invention is to provide a new and useful method and apparatus for use in an electronic article surveillance system to produce the deactivation signals which are transmitted to the tag circuit to destroy that characteristic of the tag circuit upon which its detection depends, e.g. to destroy its resonance at the search frequency to which it is sensitive.
Another object is to provide such method and apparatus which permit use of a power supply for the transmitter apparatus which is of lower power capabilities than would otherwise be required, with resultant savings in size, cost and heat-dissipating requirements of the power supply unit.
A further object is to provide such method and apparatus which is especially adapted for use in the type of system in which the frequency of the transmissions is swept over a substantial range during the search operation, but is held substantially constant during the deactivation operation, at a frequency for which the transmissions are effective to produce deactivation of the tag circuit by destruction of its resonant characteristic.